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He has never used an SLR, or SLR lens. That's all there is to it. That or he's a troll. Let this discussion of 30 against 1 (very undereducated person) die. Razr is wrong, everyone else has more than chicken noodle soup for a brain. End of story.

I normally don't insult people but you are excectional case. I'd be happily banned If you can grasp an understanding that:

For a Sigma 17-70, F/2.8-4.5, at full zoom of 70 mm the maximum aperture available is 4.5. Similar result can be expected from lenses with similar specifications. You have been wrong all along.

PS: FYI Not less than 10 guys who reply to you in this thread live and breathe photography and not than 3 are photography lecturers from colleges. As for me, I took up photography 72 hours only and I already know more about lenses than you who is a PRO. What happened?

OK, I'm really late into this thread but I had to come see what all these posts were about. I've always tried to be as polite as possible and avoid these controversies but enough is enough.

Razr, you are attacking everyone else's points

Wrong: I first posited the brutal facts about aperture priority and they, like you-have taken the opposite course.

--- but are either incapable or unwilling to define exactly where you stand.

I did so on page 1. You might go back and read what I said before you make totally gratuitous accusations.

Your posts, more and more, are taking on the life of some sort of virtual chameleon; very bizarre looking but somehow able to blend in with the background.

Casting gratuitous aspersions or damning with faith praise gets you reported. If you have something to say about my position on Aperture priority-have at it. Otherwise, stop the personal attack.

One of two things is going on. Either you are completely glib or you are completely inept when it comes to expressing yourself.

That gets you reported.

Either way, you are being unnecessarily disruptive by arguing nonsensically against some very knowledgeable people who are absolutely correct if my interpretation is correct.

Apparently you too never or seldom used or were never taught the basics of photography else you would know everything I’ve said is about Aperture priority is 100% correct.

(SNIP) If you are in fact saying that when using a f/3.5 - 4.6 lens, the widest available aperture at the short end (f/3.5) is available to the user at the long end of the same lens by using Av mode, way too many people are paying too much money for fast glass when they could just be using Av mode and you would be very, very wrong.

Since you too don’t seem to have the proper grasp of aperture priority and what is an is not happening when the lens is zoomed, try these blurbs:

Aperture Priorityhttp://www.mir.com.my/rb/photography...ry/terms_a.htm
An exposure mode on an automatic or autofocus camera that lets you set the aperture while the camera sets the shutter speed for proper exposure. If (when) you change the aperture, or the light level changes, the shutter speed changes automatically.
Apart from the sport or action arena, aperture priority is the most common & effective automatic preference in photography. It can also explained as automatic exposure system in which the lens aperture is set by the photographer, and the camera sets the shutter speed. Can be used in the stop-down mode with any lens that does not interfere with the metering system.

_________________________________________
And

http://www.picturecorrect.com/photog...epth_field.htm
Set your camera to “aperture priority”. Your camera’s manual will have simple directions on how to do this (it’s quite often just a matter of turning the dial on top). Once you have your camera on “aperture priority”, you will be able to select the aperture setting that you [bwant[/b], and the camera automatically chooses the correct shutter speed

You will note that no matter what fanciful permutations you and others use, only the shutter changes to balance the exposure.
Think of it this way: as has been posited by others, the aperture AND the shutter changes in Av when the lens is zoomed.
If that were the least bit true, the camera would be operating in “P” mode, not Av.

EDIT: I want to be completely clear here. In my above paragraph, I am asking if you are saying that I can put a 3.5-4.6 lens on my camera, let's take the 10-22 for this example, dial in an aperture value of 3.5 @ 10mm, then zoom to the long end - 22mm and in Av mode,

So far so good.

I would be able to dial in an aperture of 3.5

There is where you are confused. First, I would never make such a ghastly error as to suggest such an impossible operation.
Next, in Av, you can only set one (1) aperture value at the time.
I wonder where you got the idea you could do differently?

so the camera effectively has the same shutter speed and the aperture is letting the same amount of light in the camera at both the 10 & 22 ends? Is this what you are saying?

Never said nor inferred such madness. What you suggested here (above) cannot be done in any case: two apertures on one lens: impossible and as I noted, not what I said but what you conjured up.
Read my first post in this thread (page 1) to find out what I actually said.

Maybe your initial comment was a mistake through unwillingness to admit it, this has snowballed into a compete mess?

Not a ”mess” of my creation. The “mess” ensued from people who don’t know how cameras work in Av mode.

Either way, I invite you to clarify your argument so that maybe we can end this and some of these guys here can post in a thread more worthy of the effort.

What they (and now I understand you) need is remedial photography lessons, specifically on what happens (inside the camera/lens) during an aperture priority shot.
As I noted, go to my first post on page 1 to see what I said.

Look ... here is an "optimum" aperture shot ... that aperture was f/13

(SNIP) First, that "optimum aperture" segue was not mine. Second, you exactly- precisely proved my point about "optimum" apertures. That is, the “optimum aperture” is lens and photographer specific, though for most subjects and shooting, usually two stops down from the maximum aperture is the so-called “sweet spot”.
So for my CANON FD 55mm asph. f/1:1.2, that would be about f/1.8.

to make sure most of the object was in focus, front to back ... shot in Av Mode, with a 90mm f/2.8 lens.
(SNIP)Sorry about my choice of subjects ... how tawdry, eh?
Did I wan't f/2.8 ... no, I opted (<- operative word, here) for f/13.

The camera worked out the rest of the exposure for itself, using the built-in flash.

Had I stayed at f/2.8 ... I would get something that looks a lot like this:

What you have demonstrated with the two shots is the “optimum” aperture for that lens varied according to the purpose you decided.

Note the unwanted f/2.8 bokeh ... definitely NOT "Optimum" Aperture.

Not for that deliberately lousy set up shot, which only proves you made deliberate choices in your effort to demonstrate which aperture/lens combination works best in any given (in your case-carefully selected) shooting venue.

moving on...

Aperture_Priority_Basics

Since most of you have missed the import of aperture priority (perhaps because you never or seldom use that semi-automatic function) contesting me with your own convoluted mental and wildly off-beat mathematical calculations, I will leave this thread with this posting.

I do want you all to take note of what “aperture priority” is and what the aperture in a variable aperture does and (especially), what actually happens as a variable aperture lens set on Av is zoomed: i.e.: the camera actuates the shutter (not the aperture), making it faster or slower in order to balance the exposure: zooming in Av does not change the aperture (hole size, blade position-etc.).

Every one of these blubs below will say the same thing about aperture priority and incredibly, most of you will disagree, pointing out in your own foibles and meanly directed, widely broadcast misunderstandings of what I said and again, what the blurbs are saying.
Any sentient person can understand my “frozen in place”, “locked aperture blades” and my other informal ways to say you set the aperture (and what 100% of you have missed), the camera sets the shutter speed to balance the exposure = ”aperture priority”.
So with the aperture “locked”, the only thing that changes in “Av” mode is the shutter speed (and sometimes your ISO in order to balance the exposure at the chosen aperture.)
__________________________________________
Aperture Priority
By Vincent Bockaert

In "Aperture Priority" mode, [i]the camera allows you to select the aperture over the available range and have the camera calculate the best shutter speed to expose the image correctly. This is important if you want to control depth of field or for special effects. Note that because of their high focal length multiplier, a shallow depth of field is often very hard to achieve with digital compact cameras, even at the largest aperture.

This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
Click here to visit 123di.com

Aperture priority, often abbreviated Av or A on a camera dial, is a setting on some cameras that allows the user to choose an aperture while the camera selects a shutter speed to match. The camera will ensure proper exposure. This is different from manual mode, where the user must decide both values, shutter priority where the user picks a shutter speed with the camera selecting the aperture to match, or program mode where the camera selects both.
The main purpose of using aperture-priority mode is to control the depth of field. Aperture priority is useful in landscape photography, where a narrow aperture is necessary if objects in foreground, middle distance, and background are all to be rendered crisply, while shutter speed is often immaterial. It also finds use in portrait photography, where a wide aperture is desired to throw the background out of focus and make it less distracting.
Another common use of aperture priority mode is to suggest how the camera should decide a shutter speed, without risking a poor exposure. In landscape photography a user would select a small aperture when photographing a waterfall, hoping to allow the water to blur through the frame. When shooting a portrait in dim lighting, the photographer might choose to open the lens to its maximum aperture in hopes of getting enough light for a good exposure.
In addition, aperture priority mode allows the photographer to forcethe camera to operate the lens at its optimum apertures within the limits of maximum/minimum aperture for a given focal length of the lens. Commonly, lenses provide greatest resolving power with a relatively medium-sized aperture.
______________________________________________
Aperture Priority Modehttp://digital-photography-school.co...priority-modes
(often it has a symbol of ‘A’ or Av’ to indicate it’s selected)
In this mode you as the photographer sets the aperture that you wish to use and the camera makes a decision about what shutter speed is appropriate in the conditions that you’re shooting in.
When would you use Aperture Priority Mode? - if you remember our tutorial on aperture you’ll see how the main impact that aperture had on images was with regards to Depth of Field (DOF). As a result most people use Aperture Priority Mode when they are attempting to have some control in this area. If they want a shallow DOF (for example in the shot to the right which has the flower in focus but the background nice and blurred) they’ll select a large aperture (for example f/1.4 - as in the example to the right) and let the camera choose an appropriate shutter speed. If they wanted an image with everything in focus they’d pick a smaller aperture (for example f/22) and let the camera choose an appropriate shutter speed (generally a longer one).
When choosing an Aperture keep in mind that the camera will be choosing faster or longer shutter speeds and that there comes a point where shutter speeds get too long to continue to hand hold your camera (usually around 1/60). Once you get much slower than this level you’ll need to consider using a tripod. Also if you’re photographing a moving subject your shutter speed will impact how it’s captured and a slow shutter speed will mean your subject will be blurred).
____________________________________________http://www.earthboundlight.com/photo...-exposure.html
(SNIP) If you're game for learning how to take more control of exposure yourself, Shutter Priority and Aperture Priority are good choices. In each, you can pick one variable and the camera will pick a value for the other to yield what the camera considers to be an acceptable exposure. Thus, in Aperture Priority, you can choose any aperture you want in order to yield the depth of field you want, and the camera will pick the shutter speed. In Shutter Priority, the reverse is true. You pick the shutter speed to either freeze motion or intentionally blur it, and the camera will select an aperture to make the exposure come out what it feels is correct.
________________________________
Aperture Priorityhttp://www.mir.com.my/rb/photography...ry/terms_a.htm
An exposure mode on an automatic or autofocus camera that lets you set the aperture while the camera sets the shutter speed for proper exposure. If you change the aperture, or the light level changes, the shutter speed changes automatically. Apart from the sport or action arena, aperture priority is the most common & effective automatic preference in photography. It can also explained as automatic exposure system in which the lens aperture is set by the photographer, and the camera sets the shutter speed. Can be used in the stop-down mode with any lens that does not interfere with the metering system.
_________________________________________http://www.picturecorrect.com/photog...epth_field.htm
Set your camera to “aperture priority”. Your camera’s manual will have simple directions on how to do this (it’s quite often just a matter of turning the dial on top). Once you have your camera on “aperture priority”, you will be able to select the aperture setting that you want, and the camera automatically chooses the correct shutter speed
___________________________________________
You will note that not one blurb here (or anyplace else) mentions any foolishness about the aperture “Changing” due to this or that permutation or action like zooming.
In Av mode, the aperture (blades, size of hole-is/are) fixed and unless acted upon by human hands, are unyieldingly steadfast in their setting while the camera is in Av mode.

No one cares about Av principle right here... it's become irrelevant now. I don't care what quotes and theories you bring forth. You are wrong about the lens. My argument is that you are wrong about lenses.

You said it's possible to use a 17-70 F/2.8-4.5 lens at F/2.8 at full zoom. No pro who still think straight will say that. That's WRONG. Your attempt to divert the argument fails. Who do you teach? A bunch of sheep?

Neophyte? Getting on your nerve because of a newbie correcting you? You are dead WRONG and don't have a clue about lenses. I suggest you google for lenses for a change instead of Av.

Since most of you have missed the import of aperture priority (perhaps because you never or seldom use that semi-automatic function) contesting me with your own convoluted mental and wildly off-beat mathematical calculations, I will leave this thread with this posting.

I do want you all to take note of what “aperture priority” is and what the aperture in a variable aperture does and (especially), what actually happens as a variable aperture lens set on Av is zoomed: i.e.: the camera actuates the shutter (not the aperture), making it faster or slower in order to balance the exposure: zooming in Av does not change the aperture (hole size, blade position-etc.).

everyone gets what Ap means. what you don't understand is that Ap is limited by the lens' maximum aperture. if you are in Ap the camera will automatically adjust the aperture when using a variable aperture lens to the lens' maximum aperture without you ever touching it . try this on any camera and that is what will happen. period.

post as many lame arguments as you like, post as many links as you like but all of that information assumes that the reader has a grasp of one of the most BASIC concepts in photography and lens'. obviously you are not one of those people.

will you or will you not post the exif from a photo taken from a variable aperture lens taken at its maximum focal length to prove your absurd argument ? YES OR NO. the answer must be NO because it cannot be done.

None of the articles posted by Razr support his view. In fact, some of them even support the view expressed by all of us. It is not explicitly said that the f-stop changes if you zoom in a variable apperture lens and you have selected the maximum aperture. Why would they? There's no point in stating the obvious...

I agree with the ObiJuan on permanently banning this idiot. Razr is obviously not the professional photographer he claims he is and all he does for this site is create unrest and confuse newbies.

Then would the Great & All Knowing One please tell me how in the @%*# I can possibly set my efs 10-22 f3.5-4.5 and be able to get f3.5 when I zoom to 22mm, because I sure can't figure it out and I have had the Canon 20D almost since the day it came out. And yes, I have a couple of decades experience and my oldest brother has been doing photography since I was born (1954) and neither of us agree with your rantings.

A)You tell me how this can be done and I will apologize for what I think about you.

B)You can choose to ignore this post and continue with your impossible thinking.